1. Field of the Invention
The present invention relates to a semiconductor device, particularly a central processing unit (a CPU), for improving processing speed, in which a thin film transistor is formed over a glass substrate. The invention further relates to a method of manufacturing the semiconductor device.
2. Description of the Related Art
A conventional CPU has been fabricated by stacking semiconductor elements using a single crystal semiconductor substrate, that is, IC (integrated circuit) chips formed over what is called a silicon wafer. Such a CPU has an operation speed of more than 1 GHz, and higher frequency has been achieved in some CPUs. An integrated circuit with even higher operating frequency is expected to acheive in the future.
Active matrix type semiconductor devices including a thin film transistor formed over an inexpensive glass substrate has been developed. In particular, a technology of integrally forming an integrated circuit and a pixel area over a glass substrate to obtain a system on panel has been valued. Correspondingly, research on the formation of a large-scale integrated circuit using a thin semiconductor film has been promoted. Especially, a TFT using a polycrystalline semiconductor film has high mobility; accordingly, the TFT is expected to be a semiconductor element for fabricating a system on panel.
Some of such technologies for forming TFTs over glass substrates are shown below.
As to methods each for fabricating an insulated gate silicon semiconductor device which exhibits preferable characteristics when provided over a substrate that has low heat resistance, there is a method in which the surface of a silicon semiconductor film formed over a glass substrate with an insulating film therebetween, is exposed to an oxidizing atmosphere such as an oxygen atmosphere and further exposed to intense light to form an ultrathin oxide film over the surface of the silicon semiconductor film, and a gate insulating film is thereafter formed by plasma CVD or the like. (Reference 1: Japanese Patent Laid-Open No. 7-94756)
As another method for obtaining a high performance semiconductor device having a good ohmic contact structure, a wiring having a layered structure with more than three layers is formed and when a contact hole connected to the wiring is formed, a second conductive film of the wiring is used as an etching stopper. (Reference 2: Japanese Patent Laid-Open No. 2000-252473)
As yet another method, as to a method for forming a TFT having a GOLD structure obtained by etching a gate electrode having a first and a second layers, a resist pattern serving as an etching mask of a gate electrode is formed by photolithography, and only the second layer of the gate electrode is taper etched. The resist pattern has a finely controlled tapered angle. (Reference 3: Japanese Patent Laid-Open No. 2002-33273)
However, since a conventional CPU is formed over a silicon wafer, expensive machinery and materials are required. Further, the flexibility of the device has been poor.
Since sodium is contained in glass and glass itself has low heat resistance, a CPU fabricated using a TFT formed over a glass substrate or the like have not been obtained.
In the case of operating a CPU with high-speed, the length of a gate (gate length) of a TFT is required to be shorter. However, since a glass substrate has large deflection, a gate electrode cannot have been etched to have a gate length short enough to be used for a CPU.